Eric Hughes says:
As a consequence, our cash system is much more efficient in both computation and communication complexity than previously proposed systems."
I take this with a grain of salt; see below.
In our opinion, the most interesting of these is that the entire cash system (including all the extensions) can be incorporated straightforwardly in a setting based on wallets with observers,
I am not surprised that they find this interesting; David Chaum has patented all the observer protocols.
Having read these protocols in the original, I can say this is not much of an advantage. The observer protocols are tremendously expensive computationally. Anything you build on top of it won't be any faster.
As I understand it, "observers" are built in to supposedly uncrackable hardware built into a smart card -- and I'm not a big fan of the notion that you can make hardware non-reverse engineerable... Perry